The 20-year-old right back had already helped set up Liverpool's second goal with a pass for Georginio Wijnaldum when, with the aggregate score tied at 3-3, the ball went out behind the Barcelona goal in the 79th minute.
Alexander-Arnold, seeing the Spanish club's defenders still getting organised, feigned to walk away from the corner before racing back to whip the ball to the unmarked Divock Origi, who clipped it past Marc-Andre ter Stegen.
"I think it was just instinctive," Alexander-Arnold told BT Sport. "It was just one of those moments where you see the opportunity and obviously Div was switched on to finish it off.
"It probably came at him a bit fast but he's a top player, scored two goals for us tonight and I think it's one of them where everyone will remember this moment."
Belgian striker Origi, starting the match as part of a makeshift attack in the absence of the injured Mohamed Salah and Roberto Firmino, had triggered the Liverpool fightback with his first goal in the seventh minute.
With substitute Wijnaldum adding the other two goals inside two second half minutes, Origi said the performance had been about the collective rather than the individual.
"I think it was more about the team," he said. "We did so well, we fought and we knew it was going to be a special night. We wanted to also fight for the injured players.
"It's just special. It's hard to describe with words. You can feel it, you can hear it. Unbelievable."
Wijnaldum, who admitted to being furious at manager Juergen Klopp for being omitted from the starting line-up, came on at halftime for injured left back Andy Robertson.
"We fought so hard," Origi added. "Robbo's a fighter and we knew it could have an impact on the players. Even me at the end, I had some cramps. We did it.
"It shows we have a good mix of talent and hard work. It's special for us because we go to the final this time."
Robertson sustained his injury in a collision with Barcelona forward Luis Suarez and will have scans on his calf on Wednesday.
Liverpool's 4-3 aggregate victory earned them a spot in the June 1 Champions League final against Ajax or Tottenham Hotspur, who play their semi-final, second leg in Amsterdam on Wednesday with the Dutch side leading 1-0 on aggregate.
(Editing by Nick Mulvenney)
